1. The Field of the Invention
This invention concerns improvements in and relating to analysis of materials containing radioactive sources, particularly, but not exclusively to analysis of waste materials containing plutonium and/or other radioactive materials.
2. The Relevant Technology
When analyzing materials which potentially contain radioactive sources, it is strongly desirable to be able to accurately analyze the level of radioactive sources in the material. This is a complicated consideration as variations in the amounts and types of radioactive sources and in the amounts and types of material in which they are present, all affect the analysis. The geometric distribution of the emitters within the material constituents is also a complicating factor.
The present invention aims to provide an analysis which is more accurate and complete than previously performed analysis.
According to a first aspect of the invention we provide a method for processing information relating to gamma emissions, the method comprising the steps of obtaining an intensity signal at a plurality of gamma ray energies, the intensity at the plurality of energies being corrected for variation in attenuation with energy.
The method may form part of an investigation into the level of one or more gamma emitting materials in a sample.
The plurality of gamma ray energies may including a plurality of gamma ray energies characteristic of an isotope. The method may form part of a calculation of the level of the isotope and/or one or more other isotopes, with the calculation employing the corrected intensities.
According to a second aspect of the invention we provide a method for investigating the level of one or more gamma emitting materials in a sample, the method comprising the steps of obtaining an intensity signal at a plurality of gamma ray energies, the plurality of energies including a plurality of energies characteristic of an isotope, the intensity at the plurality of energies for the isotope being corrected for variation in attenuation with energy, the method calculating levels for the isotope and/or one or more other isotopes from the corrected intensities for the plurality of energies for the isotope.
Preferably the attenuation correction involves the calculation of a value, ideally an absolute value, for an isotope derived from the intensity at one or more characteristic energies, corrected according to a factor, the calculated value being used together with a calculated value derived from the intensity at one or more characteristic energies, including one or more different energies from the first set. Preferably a value is derived from the intensity at one energy in each case.
Preferably three or more, and ideally five or more, calculated values are employed. For Pu239 the gamma energies are preferably 98.4, 129, 203, 375 and 414 keV.
Preferably the calculated values for a set of energies obtained using the factor are considered against the calculated values for a set of energies obtained using the factor, the factor being varied between sets. Preferably the calculated values in a set are considered in a weighted manner. Most preferably those energies having a greater intensity are given a greater weighting than those having a lower intensity. The weighting may be predetermined according to the significance of the various energies used. Preferably the same energies are used in each set.
Preferably the consideration of the calculated values, most preferably in sets, involves a statistical evaluation. The statistical evaluation may involve a consideration of the difference between the calculated values or between sets thereof. The standard deviation of the calculated value for one set may be considered against the standard deviation of the calculated value for another set. Other measures of deviation may be considered. Preferably a least squares analysis is performed.
Preferably the consideration is repeated with sets corrected using different factor values. Preferably the factor values are adjusted to minimize the difference in calculated values and/or their standard deviation.
Preferably the calculated value is the mass of an isotope.
Preferably the factor includes two or more variable components. Preferably the attenuation correction is provided according to a bimodal correction factor.
Preferably one component in the correction factor relates to the attenuation effect of lower atomic mass elements (for instance less than 30, or more preferably less than 20) and/or the other component relates to the attenuation effect of high atomic mass components (for instance greater than 30, more preferably greater than 50) in the sample.
Preferably the factor is defined by:
G(Egam)=e(xe2x88x92K1.f1)*e(xe2x88x92K2.f2)
where K1 and K2 are attenuation correction fit parameters and f1 and f2 are the xe2x80x9clow Zxe2x80x9d and xe2x80x9chigh Zxe2x80x9d functions of gamma energy.
Preferably attenuation correction is provided together with detector efficiency correction and/or together with gamma line emission rate correction. Preferably all three corrections are provided. The correction factors may be applied together. Where applied separately preferably the detector efficiency and/or emission rate correction are applied before the attenuation correction.
Gamma line emission rate correction may be provided to take into account the different emission rates at different energies. The intensity at a given energy may be divided by the emission rate for that energy to give correction. The emission rate information may be obtained from a database. Detector efficiency correction may be provided to take into account non-attenuation effects which vary with energy.
Detector efficiency correction may be provided to take into account the variation in efficiency of detection of gammas at different energies within the spectrum. The intensity at a given energy may be divided by the detector efficiency for that energy to give the correction. The detector efficiency with energy profile may be obtained from manufacturers information for the detector or by investigation with known energy emission samples.
The overall correction may be provided according to the equation:       M          yx      @      y        =                              R                      y            @            y                                                ϵ                          y              @              y                                ⁢                      A                          yx              @              y                                ⁢                      ⅇ                          -              f1K1                                ⁢                      ⅇ                          -              f2K2                                          ⁢      g        -    x  
to give the mass of isotope x in grams (g-x); where Rxcex3@y is the count rate of the gamma peak for isotope x at energy y, xcex5xcex3@y is the efficiency of the detector system at energy y; Axcex3@y is the specific activity of isotope x at energy y; and exe2x88x92f1K1exe2x88x92f2K2 is a two-material attenuation model.
The attenuation correction may include correction for the contribution to an intensity value by isotopes other than the isotope under evaluation. Correction in this way may avoid too high a level being determined for that isotope. The gamma line overlap correction may be provided by deducting from the computed mass for an isotope a correction mass. The correction mass may be determined according to the equation:       M          yz      @      y        =                                                        M              yz                        _                    ⁢                      A                          yz              @              y                                                A                      yz            @            y                              ⁢      g        -    x  
to give the mass of isotope x in grams (g-x) where {overscore (Myx)} is the average mass of interfering isotope z computed using its dominant lines, interfering at energy y with isotope x. The corrected mass may then be used in the above mentioned statistical process for the attenuation factor correction. Preferably the extent of interference is recalculated in each iteration of the process. In this way feedback relating to the level of interference as the components vary can be provided.
Preferably an attenuation co-efficient for the desired energy spectrum is determined from the attenuation correction factor. Preferably the attenuation co-efficient is used to correct all the gamma intensity values used in the subsequent calculations.
The subsequent calculations may be isotopic level or mass calculations for the materials present in the sample. Absolute values may be obtained as the affects of the matrix containing the emitters have been accounted for fully. Direct comparison with calibration samples, having no attenuation effects, are therefore possible.
Preferably the isotopic calculation includes calculation of gamma emitting and/or non-gamma emitting isotopes. The level of non-gamma emitting isotopes may be determined through isotopic correlation. The level of non-gamma emitters may be derived based on a ratio relative to determined levels for one or more gamma emitters. Preferably the level is determined from a plurality of ratios to measured emitters, the level obtained from each emitter being combined, statistically, to give an overall level result.
The results obtained from the gamma emission investigations may be compared with, directly or indirectly, comparable information from other investigations. The other investigations may include passive neutron and/or active neutron monitoring techniques. The compared results may include: ratio to Pu239 gamma mass; and/or ratio to total Pu gamma mass; and/or comparison of the Pu239 gamma mass from the gamma system to the active mass from the neutron systems; and/or accounting for level of uranium, particularly U233 and U235, including ratio relative to Pu239 gamma mass, ratio of U233 to Pu239 mass, ratio of U235 to Pu239 mass, overall active mass level; and/or determination of Pu240 eff; and/or determining the presence of Cf252 and/or Cm244 and/or their levels.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.